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Problems with a Sansui QRX-9001

K

Kaleb Martin

Active Member
I have a Sansui QRX-9001 that I modified and rebuilt a couple years back (including repining the 4-ch switch board) and it has been working beautifully since. Today I turned it on and it worked properly for about 5 or so seconds then the back two channels dropped out partially. When I switch between Hall synth, Surround synth, and QS there is no change at all. SQ seems to be working correctly as well as CD-4. Every time I turn the receiver on everything works properly for around 3-5 seconds then the back two channels drop out partially again. The ICs are getting the proper 25 volt DC from the pre-power supply F-2639.

I have done quite a bit of troubleshooting (including installing known good F-2087 and F-2088 matrix and phase circuits, and it still has the same issue). I know it’s not an issue with the power output/amplifier section. This issue has me stumped.

Any help would be greatly appreciated!
 
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Did you clean the connectors (sockets) for matrix (QS) board? If not that, the issue must be in the power supply.
 
I did clean all the pins connecting the phase and matrix circuits, so it wouldn’t be that.

What in the power supply might cause an issue like this?
 
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Kaleb Martin said:
I did clean all the pins connecting the phase and matrix circuits, so it wouldn’t be that.

What in the power supply might cause an issue like this?
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I don't have a schematic so not sure is the 25V powering both boards or just matrix. Maybe some regulator is marginal and looses gain after a couple of seconds. Or a bad cap. This is probably not going to be easy to figure out.
 
The 25VDC is powering both phase and matrix circuits.

I have also replaced all electrolytic capacitors on every circuit in the unit back when I rebuilt the receiver. The only circuits I didn’t completely rebuild were the power supply and pre-power supply, as in I didn’t replace all the diodes and transistors on those boards.
 
Might want to check the relays on the quad mode switching board. There is an Omron (or equivalent) 4 pole "cube" relay, and a Hitachi 2 pole reed relay. These are in the audio signal path. The Hitachi reed relay is fairly well known for going bad. The bad news is there are no direct pin compatible replacements available for it. For that matter, the speaker protect relay ought to be checked for good signal out on all 4 channels.
 
The Hitachi relay tests good. The contacts on the Omron relay look like they could use a cleaning, but the relay pins and contacts look very fragile and difficult to clean. Is there a way of burnishing the contacts correctly short of disassembling the entire relay? And do you know if there is a modern replacement for the Omron relay?
 
The Omron relay should be readily replaceable, it's a common PC mount type with a 24 volt coil. If you can snap the clear cover off, take some paper (regular copy/printed paper soaked in contact cleaner or high grade isopropyl and run it between the contact points (will need to keep some force on the moving contact points to get enough friction to rub them clean) and see if that works.

Possible it could also be one of the (many) switches in the signal path, but I'd be surprised if you lost both rear channels simultaneously in the manner described from that.
 
And I have checked all the voltages from the power supply and pre-power supply, and they all check good.

Could this be that one of the new capacitors I installed has failed?
 
Kaleb Martin said:
And I have checked all the voltages from the power supply and pre-power supply, and they all check good.

Could this be that one of the new capacitors I installed has failed?
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I would ordinarily suspect the IC's on the card, but since you have another set of cards that's off the table. Have you checked voltages under load (cards inserted)?
 
I have checked the voltages under load and they are correct.

If I adjust the 25VDC for the ICs even slightly the back two channels will come back very briefly.
 
The QRX-9001 has issues with feedthroughs on the switch PC board (like the Dolby board in the x0x0 receivers). It san show up as many varied symptoms from intermittent channels to no sound. The switch PC board needs to be removed, and the switch assembly removed from the PC board (to access the top of some of the feedthroughs), and every feedthrough rivet needs to be soldered on both sides of the board WITH A LARGE FILLET of solder between the foil and rivet. This is a rather major job to accomplish. I believe EVERY QRX-9001 will eventually have this issue. I have repaired MANY.
 
Kaleb Martin said:
I have checked the voltages under load and they are correct.

If I adjust the 25VDC for the ICs even slightly the back two channels will come back very briefly.
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Can you check is 25VDC clean (no noise from the noisy regulator). It could also be bad trimmer.
 
AE Bob said:
The QRX-9001 has issues with feedthroughs on the switch PC board (like the Dolby board in the x0x0 receivers). It san show up as many varied symptoms from intermittent channels to no sound. The switch PC board needs to be removed, and the switch assembly removed from the PC board (to access the top of some of the feedthroughs), and every feedthrough rivet needs to be soldered on both sides of the board WITH A LARGE FILLET of solder between the foil and rivet. This is a rather major job to accomplish. I believe EVERY QRX-9001 will eventually have this issue. I have repaired MANY.
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As I said, I have repined the 4-ch switch board when I restored the unit. I used 16 AWG solid core wire in all the eyelets to make solid connections between the top and bottom layers of the board.
 
This here's a strange one. Is the 25v supply present at the two QS boards when the real channel signals drop out? I'm also wondering if the trimmer on the 25V PSU is failing, since you've started the signals come back when it's tweaked.
 
CrSens said:
Can you check is 25VDC clean (no noise from the noisy regulator). It could also be bad trimmer.
Click to expand...

I’m not certain, but I believe the 25VDC is clean (no noise), as far as I can tell with a multimeter. And when adjusting the trimmer the voltage will go up and down smoothly. I didn’t adjust it either way much though, and quickly brought it back to exactly 25 volts.
 
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Sansuiman said:
This here's a strange one. Is the 25v supply present at the two QS boards when the real channel signals drop out? I'm also wondering if the trimmer on the 25V PSU is failing, since you've started the signals come back when it's tweaked.
Click to expand...

I checked and the 25v is getting to both circuits even when the back two channels drop out.
 
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